1. Field of the Invention
The present invention relates to novel partially or totally blocked polyisocyanates with a high latent isocyanate content and high reactivity, to a process for their preparation, and to their use for producing polyurethane (PU) polymers, especially heat-curable PU coating systems, and, with particular preference, low temperature-curable PU powder coatings, and to the coatings produced accordingly.
2. Discussion of the Background
Heat-curable PU powder coatings based on partially or totally blocked polyisocyanates and hydroxyl-containing polymers, whose softening point is above 40.degree. C., are widely described in the literature, such as in the DE-A documents 21 05 777, DE-25 42 191, DE- 27 35 497 (U.S. Pat. No. 4,246,380), DE-28 42 641, DE-30 04 876, DE-30 39 824 and DE-31 28 743.
PU powder coatings consist essentially of a hydroxyl-containing component and a polyisocyanate, whose NCO groups are partially or completely masked with a blocking agent, so that the OH/NCO polyaddition reaction is unable to begin at temperatures below 140.degree. C. Only after heating them to .gtoreq.150.degree. C. is it possible to crosslink such PU powder coatings, to form a coating film within a practical period of time, with removal of the blocking agent and reaction of the OH groups with the NCO groups.
From the large number of blocking agents, described in Houben-Weyl, Methoden der organischen Chemie [Methods of organic chemistry], Volume XIV/2, 4th Edition, Georg Thieme Verlag, Stuttgart 1963, pages 61-70, only .epsilon.-caprolactam has become established in the industry for the intended use of the blocked polyisocyanates in the PU powder coating sector.
To crosslink the coating, PU powder coatings based on .epsilon.-caprolactam-blocked polyisocyanates require curing temperatures of between 170 and 200.degree. C.
There is therefore great interest in reducing the high curing temperatures so as to open up powder technology for temperature-sensitive workpieces. Likewise, of interest is the reduction of the curing times, in order to make it possible to increase the production rates (piece rates). Consequently, both environmental and economic factors are important.
Attempts have been made to achieve these aims by using oxime-blocked (cyclo)aliphatic polyisocyanates. For example, oxime-blocked polyisocyanates and their use in PU powder coatings are described in DE-A 22 00 342, EP-A 0 432 257 and U.S. Pat. No. 3,857,818. EP-B 0 401 343 describes PU powders comprising, as hardener component, an acetone oxime-blocked trimethylolpropanetetramethylxylylene diisocyanate. EP-B 0 409 745 specifies, as PU powder hardeners, 2,4-dimethyl-3-pentanone oxime- and/or 2,6-dimethyl-4-heptanone oxime-blocked isocyanurates of isophorone diisocyanate (IPDI), of methylene-bis-4,4'-cyclohexyl isocyanate and of m- and p-tetramethylsiylylene diisocyanate.
EP-B 0 531 862 relates to a process for preparing powder coatings with a glass transition temperature of 20-80.degree. C. by mixing A) a polyol component, B) a ketoxime-blocked polyisocyanate, C) a catalyst component, consisting of at least one catalyst for the reaction between blocked NCO groups and hydroxyl groups, and optionally D) further additives and auxiliaries known for powder coating technology, the powder coatings being prepared by dissolving components A, B, C and, if used, D homogeneously in an inert solvent or solvent mixture having a boiling point or boiling range between 50 and 150.degree. C., and then removing the solvent from the resulting solution.
Using oxime-blocked polyisocyanates, it is in fact possible to prepare PU powder coatings whose curing temperatures are at a low level. However, a distinction must be made between transparent and pigmented coatings. The relatively high thermal instability of such PU powder coatings is a disadvantage; the coatings have a tendency toward yellowing. A further disadvantage is the high level of susceptibility to defects ranging from pinholing to foaming, with the result that PU powder coatings containing oxime-blocked polyisocyanate are of restricted utility and can be employed only for thin-film coating.
DE-A 28 12 252 describes 1,2,4-triazole-blocked polyisocyanates which are employed in PU powder coatings--"they surprisingly bring about further improvement of the powder coating binders of the prior art". These are 1,2,4-triazole-blocked diisocyanates and/or polyisocyanates thereof which carry urethane groups.
In the description of DE-A 30 33 860 , EP 0 047 452 it is stated from page 2, line 29 to page 3, line 6 that the blocked isocyanatoisocyanurate of hexamethylene diisocyanate (HDI) is unsuitable for use in PU powder coatings. An exception is constituted by the blocked isocyanatoisocyanurate of isophorone diisocyanate (IPDI). As EP 0 047 452 goes on to show, it is possible by mixed trimerization of these two polyisocyanates (HDI/IPDI) to prepare products which, in their blocked form (cf. page 8, lines 16-21), are suitable for the PU powder coating sector, albeit with no experimental proof given. Reference is made to the variability of the melting range as a function of the HDI/IPDI molar ratios employed; increased solvent compatibility, low-temperature flexibility, etc are mentioned--cf. page 3, lines 19-21.
DE-A 33 22 718 described blocked isocyanato isocyanurates of 2-methylpentamethylene diisocyanate/2-ethylbutylene diisocyanate, and IPDI co-trimers or mixtures, HDI/IPDI co-trimers or mixtures thereof serve merely for comparison.
The object of the present invention is therefore to overcome the disadvantages of the prior art and to provide novel partially or totally blocked polyisocyanates which permit the preparation of both transparent and pigmented PU powder coatings which are notable for high reactivity, i.e., curing at low temperatures, and which therefore make it possible to obtain--with film thicknesses in the range of those encountered in practice--coatings which are free from pinholing and yellowing and are flexible despite their high network density.